1. Technical Field
The present invention relates generally to semiconductor chip packaging and carriers. More particularly, the present invention relates to a chip packaging with a cover plate that is held onto a stiffener by an interposed bonding material. The present invention also includes an alignment device for the cover plate and a method of packaging a chip.
2. Related Art
Heretofore, adhesives or mechanical structures have been used to attach cover plates to stiffeners on chip packaging. The use of adhesives create a number of problems. For instance, the use of adhesives provides poor tolerances because of uneven layering because of a failure of the adhesives to evenly or uniformly coat surfaces of a cover plate and stiffener. Precise tolerances are required because the pressures created by the adhesives being unevenly layered creates pressures on the chip, stiffener and cover plate that can create cracking. Further potential for cracking is developed by the need to repeatedly heat the package. For example, heating is required to connect the chip to the carrier, then again for connecting the stiffener and cover plate, and then again to connect the carrier to other circuitry like a printed circuit board. Poor tolerances also can create large gaps between the cover plate and stiffener resulting in poor thermal performance. To address the above problems, cover plates and stiffeners must be very accurately sized. However, this requirement adds complexity to the manufacturing process, i.e., the need to accurately form the parts and exactly locate them for joining. The need to compensate for poor tolerances also necessitates the addition of material and, hence, weight to the cover plate and the overall package.
The use of mechanical connectors encounters similar problems as to those outlined above. Another disadvantage is that mechanical connectors add more undesirable weight to the package.
For the above reasons, there is a need in the art to have a chip package, and process of manufacture therefor, where a cover plate that is more accurately attached to a stiffener to achieve lower weight and reduced potential for die crack initiation. Further, there is, as always, a need to increase thermal performance.
In a general aspect in accordance with the present invention is provided a chip package having a chip carrier, a stiffener attached to the carrier, and a cover plate attached to the stiffener by a reflowable bonding material. The bonding material is preferably a solder material. This aspect provides a number of advantages. For instance, the need to precisely size parts is greatly reduced in that the cover plate is attached to the stiffener in such a way that it self-centers itself within the stiffener when the reflowable bonding material is heated. As a result, the potential for chip cracking is reduced. Further, because the cover plate need not be so precisely sized, lower weight can be obtained for the cover plate and the overall package. There is also an increase in thermal performance not exhibited in the related art devices because of the better cover plate to stiffener connection.
The above described aspect may also advantageously include an interface material attaching the cover plate and the chip itself. This further aids in thermal conductivity and allows for setting the cover plate at an advantageous predetermined electrical potential, e.g., ground, if the interface material is electrically conductive.
In a second general aspect in accordance with the present invention is provided a chip carrier including: a stiffener, a cover plate to cover the stiffener, and an alignment device to aid in properly aligning the cover plate on the stiffener. This aspect aids in properly aligning the cover plate on the stiffener to prevent die (or chip) cracking and assure even dispersion of the bonding material.
In a third general aspect in accordance with the present invention is provided a method of packaging a chip, the packaging having a carrier, a stiffener having an opening in which the chip is positioned on the carrier, and a cover plate to cover the opening, the method comprising the steps of: providing a reflowable material between the cover plate and stiffener for attaching the cover plate to the stiffener, and attaching the cover plate to the stiffener simultaneously with an attaching of the carrier to other circuitry. This aspect provides a process of manufacturing a chip package with fewer heating cycles and, thus, the potential for reduced die (or chip) crack initiation in the package.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.